A cutout or sectionalizer is a protective device having a fuse element that is located between the high voltage power line and the distribution network grid. In the event of a fault due to a high current surge on the power line, the fuse element is designed to blow and instantly remove power from the section of the grid being protected by the cutout. This device keeps the entire grid from going down; thus, power is lost only on the section where the fault occurred.
A cutout is formed of two basic parts: (1) a fuse link holder built around an insulator and (2) a fuse assembly. The fuse assembly pivots downward after a fault current activates and blows the fuse element located within the fuse assembly. When the fuse element activates and the fuse assembly pivots downward, considerable physical force is exerted on the insulator. Hence, the insulator is typically made from porcelain or other ceramic material for added strength to prevent damage when the fuse element activates. These porcelain insulators, however, are usually heavy and bulky, require specialized assembly fixtures or processes, and are awkward to handle and ship. The porcelain insulators, being ceramic, are also brittle and easily chipped or broken.
When the fuse element of a fuse assembly activates, a lineman from a utility company needs only to see which cutout has a fuse assembly hanging in the downward position. From this he can determine which part of the network grid is faulted, locate and fix the cause of the fault, remove the fuse assembly with a hot stick, replace the fuse element inside the fuse assembly, and reinstall the fuse assembly to reenergize the cutout and once again protect the distribution network grid.
Cutouts are not per se new to the electrical distribution industry. Problems, however, have arisen with many conventional cutouts. One such problem occurs when electricity flashes directly from a conducting surface to a grounded surface while the fuse assembly is in the open or closed position. This electricity travel gap between the conducting surface and the grounded surface is called the strike distance. The strike distance in air is fairly well known, thus various size insulators are used to increase this distance.
Another problem with conventional cutouts arises when the electrical current is capable of traveling or creeping along the surface of the insulator directly from the upper end to the lower end and thereby bypass the fuse assembly. This phenomenon occurs when the insulator has an inadequate surface distance. The accumulation of water, dirt, debris, salts, and air pollutants on the insulator surface tends to provide an easier creep path for the electrical current and hence lower the effective surface distance. This surface distance is also called the "leakage," "tracking," or "creep" distance of a cutout.
Current suppliers of cutouts offer numerous insulator sizes that provide different strike and creep distances, as determined by operating voltages and environmental conditions. The higher the voltage across the conductor to be insulated, the longer the creep distance must be in order to prevent flashover or the first problem as described above. Ultimately, there is sufficient change in length to warrant the use of an entirely different length fuse assembly for various voltage classifications. Also, several different insulators with different creep distances are often provided for the same voltage classification to address the second problem as described above. These solutions to the problems, however, have still proven to be inadequate.
Cutouts with plastic or polymeric insulators have been designed in an attempt to overcome some of the failure problems incurred with conventional insulators. None of the prior plastic insulators, however, adequately solved the creep distance, material-electrical compatibility, material weatherability, and mechanical strength problems simultaneously. Some prior cutouts with plastic insulators have only attempted to solve the strength problem while inadequately addressing the creep distance. Such attempts have included plastic insulators with an insulating core material wherein the core material was selected to provide the needed strength. Examples of such cutouts with these type of insulators are U.S. Pat. No. 2,961,518 to Herman entitled "Circuit Interrupter," U.S. Pat. No. 3,868,615 to Haubein et al. entitled "Current Sensitive Interrupting Terminator Assembly," U.S. Pat. No. 4,053,707 to Ely et al. entitled "Method and Apparatus for High Voltage Insulation," U.S. Pat. No. 4,440,975 to Kaczerginski entitled "Electrical Insulator Including A Molded One-Piece Cover Having Plate-Like Fins With Arcuately Displaced Mold Line Segments," U.S. Pat. No. 4,331,833 to Pargamin et al. entitled "Insulator Comprising A Plurality Of Vulcanized Pins And Method Of Manufacture," U.S. Pat. No. 4,714,800 to Atkins et al. entitled "Stress Control/Insulating Composite Article With An Outer Surface Having Convolutions And Electric Power Cable Terminated Therewith," and U.S. Pat. No. 4,870,387 to Harmon entitled "Beam Strengthened Cutout Insulator."
Other prior cutouts attempted to solve the creep distance problem while inadequately addressing the insulator strength and weight problem, such as in U.S. Pat. No. 4,833,278 to Lambeth entitled "Insulator Housing Made From Polymeric Materials And Having Spirally Arranged Inner Sheds And Water Sheds."
Therefore, those concerned with these and other problems recognize the need for an electrical cutout that simultaneously provides the needed strength, creep distance, material-electrical compatibility, and that is also light weight for handling and shipping purposes.